1. Field of the Invention
An apparatus consistent with the present invention relates to a projection system and, more particularly, to a highly efficient projection system with an increased light efficiency which maximally uses light emitted from a light source and is made compact by using a single scrolling unit to scroll color bars.
2. Description of the Related Art
In a conventional projection system, a light valve, such as a liquid crystal display (LCD) or a Digital Micro-mirror Device (DMD), controls the on/off operation of light emitted from a light source on a pixel-by-pixel basis and forms a picture, and a magnifying projection optical system enlarges the picture to be displayed on a large screen.
Projection systems are classified into 3-panel projection systems or single-panel projection systems according to the number of light valves that are used. A 3-panel projection system provides better light efficiency than a single-panel projection system but is more complicated and expensive than the single-panel projection system. The single-panel projection system can include a smaller optical system than the three-panel projection system but provides only ⅓ of the light efficiency of the three-panel projection systems because red (R), green (G), and blue (B) colors, into which white light is separated, are sequentially used. More specifically, in the single-panel projection system, white light radiated from a white light source is separated into three color beams, namely, R, G, and B color beams, using color filters, and the three color beams are sequentially sent to a light valve. The light valve operates according to the sequence of color beams received to create images. Since the single-panel projection system sequentially uses color beams, the light efficiency is reduced to ⅓ of the light efficiency of a three-panel projection system.
A color scrolling method has been recently developed in which the light efficiency of the single-panel projection system is increased. In the color scrolling method, R, G, and B beams, into which white light is separated, are simultaneously sent to different locations on a light valve. Since an image cannot be produced until all of the R, G, and B beams reach each pixel of the light valve, the R, G, and B color beams are moved at a constant speed by a color scrolling means.
FIG. 1 is a schematic diagram of a single-panel scrolling projection system disclosed in U.S. Publication No. 2002/191154 A1. Referring to FIG. 1, white light emitted from a light source 100 passes through first and second lens arrays 102 and 104, a polarization conversion system (PCS) 105, and a condenser lens 107, and is separated into R, G, and B color beams by first through fourth dichroic filters 109, 112, 122, and 139. More specifically, the red beam R and the green beam G, for example, pass through the first dichroic filter 109 and travel along a first light path L1, while the blue beam B is reflected by the first dichroic filter 109 and travels along a second light path L2. The red beam R and the green beam G on the first light path L1 are separated by the second dichroic filter 112. The red beam R continues along the first light path L1, passing through the second dichroic filter 112, and the second dichroic filter 112 reflects the green beam G along a third light path L3.
The red, blue, and green beams R, B, and G are scrolled while passing through first through third prisms 114, 135, and 142, respectively. The first through third prisms 114, 135 and 142 are disposed in the first through third light paths L1, L2, and L3, respectively, and as the first, second, and third prisms 114, 135, and 142 rotate at a uniform speed, R, B, and G color bars are properly scrolled. The blue and green beams B and G, which travel along the second and third light paths L2 and L3, respectively, are transmitted and reflected by the third dichroic filter 139, respectively, and combined. The red, green, and blue beams R, G, and B are then combined by the fourth dichroic filter 122. The combined beam is transmitted by a polarization beam splitter (PBS) 127 and forms a picture using a light valve 130.
The scrolling of the R, G, and B color bars due to rotation of the first through third prisms 114, 135, and 142 is illustrated in FIG. 2. Scrolling represents the movement of color bars formed on the surface of the light valve 130 when the first, second, and third prisms 114, 135, and 142 corresponding to R, B, and G colors, respectively, are synchronously rotated.
A color image obtained by turning the pixels of the light valve 130 on or off according to an image signal is magnified by a projection lens (not shown) and projected onto a screen.
Since the conventional projection system uses different light paths for different colors, a light path correction lens must be included for each of the colors, components for unifying the separated light beams must be further included, and separate components must be included for each of the colors. Hence, the conventional optical system is bulky, and the manufacturing and assembly thereof is complicated, thus decreasing the yield.
Three motors for rotating the first, second, and third scrolling prisms 114, 135, and 142 generate much noise during operation. Thus, the projection system adopting three motors is manufactured at a greater cost than a color wheel type projection system which utilizes a single motor.
In order to produce a color picture using a scrolling technique, color bars as shown in FIG. 2 must be scrolled at a constant speed. Hence, the conventional projection system must synchronize the light valve 130 with the three prisms 114, 135, and 142 in order to achieve proper scrolling. However, controlling the synchronization is not easy. Due to the circular motion of the scrolling prisms 114, 135, and 142, the color scrolling speed by the three scrolling prisms is irregular, consequently deteriorating the quality of the resultant image.
FIG. 3 is a schematic diagram of an illumination system included in the conventional projection system of FIG. 1. Referring to FIG. 3, an unpolarized beam is radiated from a lamp 111 of the light source 100 and reflected by a reflection mirror 113. The reflection mirror 113 reflects the unpolarized beam emitted from the lamp 111 so that the unpolarized beam travels along a light path. The unpolarized beam reflected by the reflection mirror 113 is divided into a plurality of beams by the first lens array 102. The divided beams are focused in front of the second lens array 104. The first and second lens arrays 102 and 104 may be either cylindrical lens arrays or fly-eye lens arrays.
The focused beam passes through the second lens array 104 and is incident upon the PCS 105. The PCS 105 polarizes the incident beam and includes first and second PBSs 123 and 124, which are disposed perpendicular to the direction of the incident beam, and a ½ wavelength plate 122, which is adjacent to the first and second PBSs 123 and 124 and changes the polarization direction of the incident beam.
The first PBS 123 transmits a first beam having one polarization from an unpolarized beam received from the second lens array 104 and, at the same time, reflects a second beam with the other polarization toward the second PBS 124. To achieve this, the first PBS 123 includes a first polarization filter 123a. When the first PBS 123 receives an unpolarized beam, the first polarization filter 123a transmits a P-polarized beam (shown as parallel to the paper) and reflects an S-polarized beam (shown as perpendicular to the paper).
The second PBS 124 re-reflects the second beam received from the first PSB 123 and the second beam becomes parallel to the first beam transmitted by the first PSB 123. To achieve this, the second PBS 124 includes a second polarization filter 124a. 
The ½ wavelength plate 122 changes the second beam reflected by the second polarization filter 124a from an S-polarized beam to a P-polarized beam like the first beam.
The PCS 105 increases the light efficiency by converting an unpolarized incident beam into a beam with a single polarization. However, the PCS 105 with the above-described structure generates a beam loss because of cell boundaries of the first and second lens arrays 102 and 104. Furthermore, the structure of the PCS 105 is complicated.